1. Field of the Invention
This invention relates to the control of automatic transmissions for automotive use. More particularly, the invention pertains to hydraulic controls that produce the operating range selected by the vehicle operator.
2. Description of the Prior Art
In the operation of an actuation and control circuit of an automatic transmission, various friction elements of the transmission, i.e., hydraulically actuated clutches, brakes and servos, are engaged and released to produce the gear ratios of the forward drive range and reverse drive range. Hydraulic fluid, at elevated pressure, is supplied to several friction elements, each having a hydraulic piston located within a cylinder, thereby causing frictional contact among stacked friction plates connected to the driving and driven members.
When the friction element is disengaged during a gear ratio change, the torque capacity of the friction element is lowered by venting hydraulic fluid from the cylinder to a low pressure sump located below the circuit.
In a conventional device for selecting the operating range of an automatic transmission, movement of the gear selector lever by the vehicle operator is transmitted by a mechanical linkage to a manual valve, a component in the actuation and hydraulic control circuit that is supplied with regulated line pressure and directs line pressure selectively to output ports of the manual valve. A spool of the manual valve moves in accordance with the corresponding position of the gear selector lever to which it is mechanically connected. For example, if the gear selector is moved to a drive range (D or OD), the manual valve connects the regulated line pressure source to an output port through which all of the passages of the hydraulic circuit required to be pressurized to produce the selected gear ratios are pressurized.
When the gear selector is moved from a forward drive position to the reverse position, the manual valve closes the output port that supplies pressure to the passages requiring pressurization in the forward drive condition, and instead connects the line pressure source to an output port that pressurizes the lines required to be pressurized to produce reverse drive. When the gear selector is moved to the forward drive position from the reverse drive position, the manual valve closes the output port that supplies pressure to the passages requiring pressurization in the reverse drive condition, and instead connects the line pressure source to an output port that pressurizes the lines required to be pressurized to produce forward drive. When the gear selector is moved to the neutral position, the manual valve closes the output ports of the manual valve.
Modern automatic transmissions are increasingly using electronic controls to provide improved transmission function. Electronic controls are being used to control transmission line pressure, shift scheduling, and shift quality. This invention relates to using electronic controls for selecting the range, i.e., forward, reverse, and neutral. Electronic range selection would replace the conventional manual control valve and associated internal and external linkage.